Diode capacitors are used in a wide range of circuits. Nonlinear response to a variable voltage condition limits the usefulness of diode capacitors in certain applications. The nonlinearity may have to be addressed with supplemental circuitry in some cases. In other cases, the nonlinearity may limit the operational range of the circuit.
As an example, voltage variable (or tunable) diode capacitors are used extensively in high-frequency electronic design applications. Cellular handset circuits, for example, use voltage variable diode capacitors in voltage controlled oscillators that are used for tuning. Voltage variable diode capacitors are used in a wide variety of additional circuits, including, for example, tunable filters and high-frequency switches. Their utility and performance are compromised as the capacitance of the diodes used to construct the variable voltage capacitors varies nonlinearly with the voltage applied to the capacitor. With higher signal levels, the nonlinearity creates inter-modulation and cross-modulation distortion. Such distortion can limit circuit performance.
In R. G. Meyer and M. Stephens “Distortion in Variable Capacitance Diodes,” IEEE J. of Solid-State Circuits, vol. SC-10, no. 1, 1975, variable capacitor diodes are discussed. The paper discusses the anti-parallel connection of two varactor (voltage variable reactance) diodes to realize a composite voltage variable capacitor with improved linearity. In a special case referred to as the abrupt junction case, nonlinearities are cancelled, and the resulting configuration is almost perfectly linear. However, nonlinearity cancellation is less effective outside of the abrupt junction special case, minimizing the general utility of the technique. The abrupt junction case requires maintenance of a large dc bias voltage range to realize a large capacitance variation.
Variable diode capacitors are used in RF and microwave circuits, for example. Such circuits also use matching networks. Reducing loss with adaptive matching networks holds promise for the further advancement of RF and microwave circuits. For example, low loss adaptive matching networks can be used to tune the matching conditions for power amplifiers to dynamically optimize the load impedance, in order to provide the best performance at varying output powers and antenna conditions. Other example uses of adaptive passive networks are for tunable filters, multi-band radios, and reconfigurable RF systems.
Typical variable capacitance circuits use varactor diodes for tuning. Applications of these circuits include voltage controlled oscillators, tunable filters, switches, phase shifters and tunable impedance matching networks. Achieving low-distortion in these circuits is highly desirable—especially for large-signal applications like power amplifiers.
Some alternative strategies for variable capacitance circuits are being researched. One example are capacitors based upon microelectromechanical systems (MEMS). MEMS capacitors provide a very high Quality Factor (Q) and extraordinarily high linearity, but require non-standard processing and packaging techniques, and high control voltages. Additionally, and their reliability and switching speed are still poor compared to semiconductor-based solutions. Other proposed tuning techniques, based on voltage-variable dielectrics, exhibit similar drawbacks of manufacturability and performance.
In view of such integration issues, varactor diodes seem a logical choice for implementing RF adaptivity. However, their inherently nonlinear behavior disqualifies them for use with modern communication standards characterized by high peak-to-average power ratios, and their related Q factors are usually too low at the microwave frequencies of interest for the most demanding applications.
In particular, next-generation wireless systems, such as multi-mode transceivers and “cognitive radios,” require circuit techniques that facilitate RF adaptivity. Some examples of adaptive circuits include tunable filters, tunable matching networks for low-noise and power amplifiers, and multi-band VCO's. An ideal tuning element for these applications will exhibit extremely low loss, low dc power consumption, high linearity, ruggedness to high voltage and high current, wide tuning range, high reliability, very low cost, low area usage, and be continuously tunable, with a high tuning speed.
PIN Diodes or GaAs pseudomorphic high electron mobility transistors (PHEMTs) are commonly used today for these challenging applications. However, these solutions are considered to be too expensive, or to consume too much dc power, to be an acceptable long term solution for cost and performance sensitive applications.